1. Field of the Invention
The present invention relates to an optical glass and to an optical element formed thereof. More particularly, the invention relates to an optical glass suitable for press-molding and to an optical element formed of such an optical glass.
2. Description of Related Art
One method of fabricating a glass optical element is so-called press-molding, whereby molding is achieved by pressing glass heated to above the yielding temperature (At) thereof between a pair of heated metal molds—an upper and a lower mold. This involves less fabrication steps than other conventional molding methods that involve polishing of glass, and thus allows fabrication with less time and at lower costs. Press-molding has therefore come to be widely employed in recent years to fabricate optical elements such as glass lenses.
There are mainly two types of press-molding: one involving reheating and the other involving direct-pressing. In press-molding of the reheating type, a gob preform or a polished preform having largely the shape of the finished product is prepared, and this preform is then reheated to above the softening point and press-molded between a pair of heated metal molds—an upper and a lower mold—so as to be formed into the shape of the finished product. On the other hand, in press-molding of the direct-pressing type, drops of molten glass from a glass melting furnace are dropped directly into a heated metal mold and is press-molded so as to be formed into the shape of the finished product. In press-molding of either type, when glass is molded, the pressing metal mold needs to be heated to near or above the glass transition temperature (hereinafter also represented by “Tg”). As a result, the higher the Tg of glass, the more prone the pressing metal mold is to surface oxidation and to alteration in the metal composition, and thus the shorter the useful life of the pressing metal mold, resulting in higher fabrication costs. Deterioration of the metal mold may be alleviated by performing molding in an atmosphere of an inert gas such as nitrogen. The control of the atmosphere, however, requires a complicated design in molding equipment, and the use of the inert gas incurs a running cost, leading to higher fabrication costs. Thus, it is desirable that glass used in press-molding have as low a Tg as possible. In addition, from the viewpoint of enhanced resistance to devitrification, it is desirable that not only the Tg but also the liquid phase temperature (hereinafter also represented by “TL”) of the glass be as low as possible.
Accordingly, various technologies for lowering the Tg and TL of glass without using a lead compound have been studied and proposed, of which examples are disclosed in JP-A-2005-330154, US 2005/0204776 A1, and JP-A-H06-305769.
Inconveniently, however, the optical glasses proposed in the documents mentioned above achieve a low Tg and a low TL through addition of large amounts of alkali ingredients, and thus have low viscosity, for example, at the nozzle temperature during dropping of glass for direct press-molding. This often makes it difficult to drop the glass through the nozzle, and makes the dropped glass lie flat on the metal mold, making it difficult to mold.